Invasion in a brackish marsh: Lepidium latifolium impacts invertebrate community structure and function

Background/Question/Methods

Over the last two decades, human-induced habitat changes have degraded 90% of California’s coastal wetlands. Lepidium latifolium (perennial pepperweed) is a pervasive invader of California wetlands, potentially altering ecological and community dynamics.  This project assesses L. latifolium’s impact on community structure of two invertebrate groups in the brackish marsh of Rush Ranch Open Space Preserve, a component of the San Francisco Bay-Delta Estuary. We sampled both soil-dwelling infauna and canopy-dwelling insects in order to understand L. latifolium’s impact on the invertebrate community as a whole. Samples were taken in all four seasons utilizing a paired design which sampled equally between L. latifolium invaded and non-invaded marsh areas in three major geomorphic units of the marsh (fringing tidal marsh, tidal marsh plain, and tidal marsh-terrestrial ecotone).  Soil-dwelling infauna were collected using 2 cm sediment cores, and canopy-dwelling insects were gathered using a leaf blower modified to intake air (sampling time 30 sec to 1 min). Samples were sorted and individuals were identified to species or morphospecies, and categorized by trophic feeding mode.  PRIMER statistical software and PERMANOVA were utilized to examine changes in invertebrate abundance, species richness, diversity, and community similarity both among and within seasons.

Results/Conclusions

Abundance, richness, diversity, and community similarity differed significantly among seasons for both soil-dwelling infauna and canopy-dwelling insects.  Within season, pant type (invaded vs. non-invaded) only impacted infaunal abundance in the fall through a significant interaction between geomorphic unit and plant type, which increased abundance in the invaded portions of the tidal marsh-terrestrial ecotone (PERMANOVA, Pseudo-F=3.9998, p(MC)=0.04004).  For canopy-dwelling insects impacts of LELA were observed in all seasons. In the fall, richness of trophic modes was elevated in invaded areas of the marsh (PERMANOVA, Pseudo-F=9.00, p(perm)=0.013), and in the spring, trophic community structure varied by plant type (ANOSIM, Global R=0.252, p=0.032).  In summer abundance and species richness were greater in invaded areas of fringing tidal marsh (abundance paired PERMANOVA, FTM_{invaded vs. nat}, t=0.3389, p(MC)=0.0003; richness paired PERMANOVA, FTM_{invaded vs. nat}, t=158.18, p(MC)=0.0002), and in the winter, abundance was significantly higher in the non-invaded marsh (PERMANOVA, Pseudo-F=12.565, p(MC)=0.006.  We believe L. latifolium’s unique plant traits (large inflorescence, shifting habitat structure), increases insect abundance while in bloom, but offers sub-optimal habitat once the plant has senesced.  We hypothesize that infauna is little impacted by this invasion.  Looking at these two groups concurrently has allowed us to understand the variable impacts of this invasion.